1. Field of the Invention
The present invention relates to a DC-DC converter for converting a voltage of a DC (Direct Current) power supply, and more specifically, it relates to an improvement in an output low voltage protection function which stops supply of power when a short circuit occurs in the load.
2. Description of the Related Art
A conventional DC-DC converter is explained according to FIG. 4. FIG. 4 is a circuit diagram showing an example of a conventional DC-DC converter unit. Incidentally, an explanation is made by using a flyback converter as an example. As shown in FIG. 4, the conventional DC-DC converter consists of a DC power supply 1, a transformer 2, a PWM (Pulse Width Modulation) modulation section 40, a power supply selection section 41, a time constant determination section 42, a thyristor 21, a first rectifier section 43, a first smoothing section 44, a photocoupler 19, a shunt regulator 22, dividing resisters 23 and 24, a second rectifier section 45, and a second smoothing section 46.
Furthermore, the PMW modulation section 40 consists of a PWM control IC 4 and a main switch 3. The PMW control IC 4 is provided with a power supply terminal (Vcc), a signal output terminal (OUT), an operation control input terminal (OFF), and a GND terminal. The power supply selection section 41 consists of a first FET (N channel MOSFET) 5, a first resistor 6, a second resistor 7 and a Zener diode 8. The time constant determination section 42 consists of a resistor 18 and a capacitor 20. The first rectifier section 43 is provided with two diodes 9 and 10. The first smoothing section 44 is provided with a choke coil 11 and a capacitor 12. Moreover, the second rectifier section 45 is provided with two diodes 14 and 15. The second smoothing section 46 is provided with a choke coil 16 and a capacitor 17. Incidentally, in this case, the main switch 3 consists of an FET (hereinafter referred to as a second FET for discrimination).
In a first FET 5 in the power supply selection section 41, the drain terminal (D) and the gate terminal (G) are respectively connected to a positive pole side of a DC power supply 1 via the first resistor 6 and the second resistor 7. The gate terminal (G) of the first FET 5 is also connected to a cathode terminal (K) of the Zener diode 8. An anode terminal (A) of the Zener diode 8 is connected to a negative pole side (hereinafter referred to as xe2x80x9cGNDxe2x80x9d). Moreover, a source terminal (S) is connected to the power supply terminal (Vcc) of the PMW control IC 4 and to an output terminal Q of the second smoothing section 46.
A power supply terminal of the PWM control IC 4 of the PWM modulator 40 is connected to a terminal X of the photocoupler 19 via the resistor 18. The terminal X of this photo-coupler 19 is also connected to a gate terminal (G) of the thyristor 21 and is connected to GND via the capacitor 20. Moreover, a terminal Y of the photocoupler 19 is connected to GND. The anode terminal (A) and the cathode terminal (K) are connected to the operation control input terminal (OFF) of the PWM control IC 4 and GND respectively. The signal output terminal (OUT) of the PWM control IC 4 is connected to the gate terminal (G) of the second FET 3 and the GND terminal of the PWM control IC 4 is further connected to GND. Incidentally, a feedback signal of the output voltage is input to this PWM control IC 4 by an output voltage detection circuit and a feedback circuit which are not illustrated. A source terminal (S) of the main switch 3 is connected to GND and a drain terminal (D) thereof is connected to the positive pole side of the DC power supply via a primary winding 2a of a transformer 2.
Both ends of a secondary winding 2b of the transformer 2 are connected to a load 13 via the first rectifier section 43 and the first smoothing section 44, a terminal P side of this load 13 is connected to the anode terminal of the LED of the photocoupler 19, the cathode terminal of the photocoupler 19 is connected to the cathode terminal (K) of the shunt regulator 22. Moreover, the anode terminal (A) of this shunt regulator is connected to the other end of the load 13, a reference terminal (R) is connected to the terminal P side of the load 13 via the resistor 23 and is connected to the other end of the load 13 via the resistor 24.
Both ends of the third winding 2c of the transformer 2 are connected to the second smoothing section 46 via the second rectifier section 45. Incidentally, GND terminals of the second rectifier section 45 and the second smoothing section 46 are connected to GND respectively as they are shown.
Operations of the PWM modulation section 40 and the power supply selection section 41 are now explained. In the PWM modulation section 40, the PWM control IC 4 controls conduction of the main switch 3 by a PWM-controlled output signal to control the current flowing to the primary winding 2a of the transformer 2. Moreover, the PWM modulation section 40 stops output of the PWM-controlled signal when the operation control terminal (OFF) of the PWM control IC 4 is set to an L-level.
Furthermore, when the second smoothing section 46 does not supply sufficient sub-power supply voltage (when starting), the power supply selection section 41 turns the first FET 5 ON to supply power supply voltage of the DC power supply 1 supplied via the resistor 6 to the power supply terminal (Vcc) of the PWM control IC 4. When the second smoothing section 46 supplies sufficient sub-power supply (in a steady state), the power supply selection section 41 turns the first FET 5 OFF to supply the sub-power supply voltage outputted from the second smoothing section 46 to the power supply terminal (Vcc) of the PWM control IC 4 as it is shown.
Incidentally, the time constant determination section 42 is widely known, and a choke-input rectifying method which is well known is utilized for the first rectifier section 43, the first smoothing section 44, the second rectifier 45 and the smoothing section 46. Therefore, detailed explanations are omitted.
Next, operations of the conventional DC-DC converter shown in FIG. 4 are explained. First, an operation of this DC-DC converter when starting is explained. The voltage from the DC power supply 1 is applied to the gate electrode (G) of the first FET 5 via the resistor 7, and the gate electrode (G) is fixed at predetermined potential to GND by function of the Zener diode 8. On the other hand, since the voltage is not applied to the source terminal (S) of the first FET 5 at first, a potential difference is generated between the gate and source terminals to turn the first FET ON, and the voltage to be applied to the drain terminal (D) of the first FET 5 via the resistor 6 is transmitted to the power supply terminal (Vcc) of the PWM control IC 4 via the source terminal (S). Then, the PWM control IC 4 is turned ON to start, and a PWM modulated voltage signal (a gate driving pulse) is outputted from the output terminal OUT. Incidentally, in this case, the PWM control IC 4 is fed back to be inputted with output voltage. Thereby, the PWM control IC 4 performs operations to control the output voltage to a predetermined value.
Subsequently, the main switch 3 is switching-controlled by the gate driving pulse outputted from the PWM control IC 4, and a current between the drain electrode (D) and the source electrode (S), that is, the current flowing through the primary winding 2a of the transformer 2 from the DC power supply is converted to an AC current.
Then, a current in accordance with the number of wire turns is induced in the secondary winding 2b of the transformer, and a current in accordance with the number of wire turns is also induced in the tertiary winding 2c. A voltage signal generated on both ends of the secondary winding 2b of the transformer 2 is rectified by the first rectifier section 43, then, it is smoothed by the first smoothing section 44 and to be applied to a load 13 as a DC voltage signal.
On the other hand, the voltage signal generated on both ends of the tertiary winding 2c of the transformer 2 is converted to DC current by the second rectifier section 45 and the second smoothing section 46, and is supplied to the supply terminal (S) of the first FET 5 via a terminal Q as sub-power supply voltage. In this case, the sub-power supply voltage has a size that is proportional to the voltage applied to the load 13.
After the starting operation has been completed as explained above, since the sub-power supply voltage is applied to the source terminal (S) of the first FET 5, the potential difference between the source electrode (S) and the gate electrode (G) becomes zero, the first FET 5 is turned OFF. In other words, each circuit constant is decided so that the first FET 5 is turned OFF by a DC current that appears on the terminal Q when a predetermined voltage is applied to the primary winding 2a of the transformer 2. On the other hand, immediately after the first FET 5 has been turned OFF, since the DC current appearing on the terminal Q is supplied to the power supply terminal (Vcc) of the PWM control IC 4, the PWM control IC 4 keeps outputting the PWM modulated signal without any stop in operation. In this way, the PWM control IC 4 obtains a power supply from the DC power supply 1 only at the time of starting, and is supplied with power from the terminal Q during steady state operation. Thereby, conduction loss is prevented.
When the circuit comes to steady state operating an LED of the photocoupler 19 is turned ON by operations of dividing resistors 23 and 24 and the shunt regulator 22, then the terminal X is brought to conduction with the terminal Y. As a result, the voltage between the gate and cathode of the thyrister 21 comes to the L-level, and thyristor 21 goes to the L-level, and thyristor 21 is maintained in an OFF state.
In this case, if the voltage applied to the load 13 decreases because of a short circuit or the like of the load 13, the shunt regulator 22 causes cut off of conduction with the photocoupler 19, and the terminal X will not conduct with the terminal Y. After a predetermined time determined by the time constant circuit 42 has elapsed, the voltage between the gate and cathode of the thyristor 21 goes to the H-level, and the thyristor is turned ON. Then, a stop signal input terminal of the PWM control IC 4 comes to L-level, and the switching operation of the PWM control IC 4 is latched (a conversion operation to AC is stopped and the stop state is maintained). Thus, protection of the output low voltage is realized. Incidentally, although a state in which conduction between the photocoupler 19 and a short time the LED can be cut off for, in this case, the thyristor 21 is never turned ON, the switching operation of the PWM control IC 4 is prevented from latching by the time constant circuit.
However, in the conventional DC-DC converter mentioned above, since parts of large size and high cost such as the photocoupler and shunt regulator are employed to realize protection of the output low voltage, there have been problems in that miniaturization was difficult and the cost could not be reduced.
The present invention is made considering the above-mentioned situation, and it is the purpose of the present invention to provide a DC-DC converter which can be miniaturized and which reduces cost while realizing protection of the output low voltage.
To solve the conventional problem mentioned above, the invention comprises a DC-DC converter having:
a primary-side circuit connecting to a DC power supply source and provided with a converter for converting voltage supplied by said DC power supply into an AC voltage signal,
a secondary-side circuit for rectifying and smoothing the AC signal transmitted from the primary-side circuit to supply to a load and auxiliary rectifying and smoothing circuit, the primary-side circuit further having a monitor for monitoring a sub-power supply voltage supplied from the auxiliary rectifying and smoothing circuit to the converter, and a latch for latching a conversion operation of the converter.
As described above, by utilizing a state in which the sub-power-supply voltage relates (for example, is proportional to) to the output voltage of the secondary-circuit side, by monitoring this sub-power supply voltage, that is, the power-source voltage in the primary-side circuit, a decrease in output voltage caused by short circuit of the load is detected to stop a conversion operation without employing an element having a large size such as a photo-coupler. Thus, miniaturization and cost reduction can be achieved while realizing protection of the output low voltage.
Furthermore, to solve the conventional problem mentioned above, the invention further includes, in the DC-DC converter, a primary-side provided with a DC power supply and a converter for converting voltage supplied by the DC power supply into an AC voltage signal and an auxiliary rectifying and smoothing circuit for rectifying and smoothing the AC signal transmitted from the primary-side circuit to output as a sub-power supply voltage, and having a power supply selection circuit for supplying the power supply voltage from the DC power source when the converter starts and selectively supplying the sub-power supply voltage from the auxiliary rectifying and smoothing circuit in a steady state and a stop circuit for monitoring the sub-power supply voltage to latch the operation of the converter when the sub-power supply voltage decreases.
Moreover, to solve the conventional problem mentioned above, in the DC-DC converter has an operation controller provided with an operation control terminal for controlling stopping of the conversion operation of the converter according to signals inputted to the operation control terminal and preventing latching during the controlling stopping of the conversion operation. Thereby, the DC-DC converter can be remote-controlled without being latched.
Moreover, to solve the conventional problem mentioned above, the DC-DC converter has an input low voltage protection circuit for monitoring the power supply voltage to control stopping of the conversion operation of the conversion circuit when the power supply voltage is lower than a predetermined voltage and for preventing latching during the controlling to stop. Thereby, protection of the input low voltage can be realized without being latched.
Moreover, to solve the conventional problem mentioned above, the converter is provided with a pulse-width conversion control element comprising a power supply terminal, a control terminal for stop controlling a conversion operation of pulse-width conversion and a terminal for inputting a signal whose pulse width is converted and a main switch element for receiving the signal whose pulse width is converted in the gate terminal to allow the drain terminal to conduct with the source terminal according to the signal whose pulse wave is converted, the power supply selection circuit includes a first transistor provided with a control input terminal which is fixed to a predetermined potential to ground potential, an input terminal supplied with the power-source voltage of the DC power supply and an output terminal supplied with the sub-power supply voltage, the first transistor allowing the input terminal to conduct with the output terminal to supply the power-source voltage from the DC power supply to the power supply terminal of the converter via the output terminal before the sub-power supply voltage reaches the predetermined potential and supplies the sub-power supply voltage to the power supply terminal of the converter without making the input terminal conduct with the output terminal when the sub-power supply voltage exceeds the predetermined potential, the stop circuit including a second transistor for sending a signal for stop control to the control terminal of the converter upon receiving the current signal at the input terminal of the first transistor.
According to the present invention, since in the DC-DC converter the sub-power supply voltage outputted by the auxiliary rectifying and smoothing circuit is monitored at the primary-side circuit side, when the monitor detects a decrease in the sub-power supply voltage, conversion operation to AC current is stopped, and the state of the secondary-side circuit is not needed to be transmitted to the primary-side circuit. As a result, by employing small-sized and low cost elements, miniaturization and cost reduction can be achieved while realizing protection of the output low voltage.
Furthermore, the DC-DC converter is constituted so that when a conversion circuit which is provided in the primary-side circuit and converts voltage from the DC power supply to an AC voltage signal is supplied with power supply voltage by the DC power supply when starting and is supplied with sub-power supply voltage by the auxiliary rectifying and smoothing circuit, the stop circuit monitors the sub-power supply voltage to stop conversion operations of the conversion circuit when the sub-power supply voltage decreases. Therefore, there is no need to transmit a state of the secondary-side circuit to the primary-side circuit. Thus, miniaturization and cost reduction can be achieved while realizing protection of the output low voltage by employing low cost elements.
Furthermore, the DC-DC converter unit is constituted so that an operation control circuit stop-controls conversion operations of a conversion circuit in accordance with a signal inputted to an operation control input terminal, and latching is prevented during a stop operation. Thus, the DC-DC converter unit can be remotely controlled without causing latch stop.
Furthermore, the DC-DC converter is constituted so that the input low voltage protection circuit monitors the power supply voltage to stop-control conversion operations of the conversion circuit, and latching is prevented during a stop operation. Thus, protection of the input low voltage can be realized.
Furthermore, the DC-DC converter is constituted so that the stop circuit has a structure which is realized by a transistor; thereby, miniaturization and cost reduction can be achieved while realizing protection of output low voltage.